Join force in a predicament (5)
I believe the answer is:
affix
'join' is the definition.
(I've seen this before)
'force in a predicament' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'in' means one lot of letters goes inside another.
'predicament' becomes 'fix' (I've seen this in another clue).
'a'+'fix'='afix'
'f' inserted within 'afix' is 'AFFIX'.
(Other definitions for affix that I've seen before include "Append" , "Attach" , "Join" , "Glue" , "Fasten (to something)" .)