Inflexible barrier installed by a worker (7)
I believe the answer is:
'inflexible' is the definition.
(similar in meaning)
'barrier installed by a worker' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'installed by' means one lot of letters goes inside another.
'worker' becomes 'ant' (worker is a type of ant).
'dam' going within 'aant' is 'ADAMANT'.
(Other definitions for adamant that I've seen before include "Positive and unrelenting" , "Insistent, inflexible" , "Unshakable (in an opinion)" , "Unswerving" , "6a [PRINCESS IDA] 's fortress < for unyielding" .)