Tight-head showing nerves (9)
I believe the answer is:
tenseness
'nerves' is the definition.
(tenseness is a kind of nerves)
'tight-head' is the wordplay.
'tight' becomes 'tense' ('tense' can be similar in meaning to 'tight'**).
'head' becomes 'ness' (ness is a headland).
'tense'+'ness'='TENSENESS'
'showing' acts as a link.
(Other definitions for tenseness that I've seen before include "Cosmetic lotion" , "Condition of being taut" .)