Growth of power with steel tower mostly erected (5)
I believe the answer is:
polyp
'growth' is the definition.
(polyp is a kind of growth)
'power with steel tower mostly erected' is the wordplay.
'power' becomes 'p'.
'with' means one lot of letters go next to another.
'steel tower' becomes 'pylon' (I've seen this before).
'mostly' means to remove the last letter.
'erected' is a reversal indicator (in a down clue, letters go up).
'pylon' with its last letter removed is 'pylo'.
'pylo' in reverse letter order is 'olyp'.
'p'+'olyp'='POLYP'
'of' acts as a link.
(Other definitions for polyp that I've seen before include "Minute reef-building creature" , "Sea anemone, perhaps" , "Sea anemone, for example" , "Sea creature; small growth" , "Small growth, usually benign" .)